Some radio transmission standards such as, the Bluetooth standard, specify a 2.4 GHz frequency-hopped, spread-spectrum system using a Gaussian-FSK (GFSK) modulation scheme with a data rate of 1 Mb/sec. The frequency offsets are ±160 kHz, corresponding to a nominal modulation index of 0.32 and the data bandwidth is about 1 MHz. Consequently, an exemplary Bluetooth receiver has a 2 MHz intermediate frequency (IF), causing the image signal to be within the 80 MHz ISM band. The image reject requirement is therefore relaxed, and may be achieved by an on-chip complex-domain bandpass filter (BPF). Once the desired channel is selected, a limiter amplifies it to a well-defined level, and the received signal strength is indicated.
A commonly used type of pulse-code modulation is FSK, in which the carrier frequency changes abruptly by an amount Δf as the modulating signal changes from the zero level to the one level, or vice versa. A FSK demodulation typically uses a differentiator followed by I and Q multipliers. However, if implemented at base-band, the desired signal at 2 MHz is down-converted to zero IF. This approach needs to use four additional mixers for single side-band, quadrature downconversion, and a 2 MHz clock generator. This increases the receiver power dissipation, and leads to a larger silicon area. Additionally, the base-band demodulator is more susceptible to DC offset and low frequency noise and interference.
Typically, an analog differentiator followed by an envelope detector can be used to detect an FM modulated signal. This is simple and low-power, however, its performance is limited due to the inaccuracy of the components once realized in an IC. In contrast, an FSK demodulator may be implemented digitally, which is more robust, yet it consumes more power, and is more complex.
Moreover, a direct-conversion receiver also has few disadvantages. For example, since the GFSK spectrum has energy at zero IF, DC offset and 1/f noise may degrade the receiver performance. Also, a limiter at base-band is generally not suitable for a GFSK signal down-converted to zero IF, since the harmonics of the limited signal fall inside the desired band, degrading the receiver BER. This problem may be evaded if the limiter is substituted by an automatic gain controller (AGC). However, an AGC is complicated and consumes more power.
Therefore, there is a need for a radio transceiver having the characteristics of low size, cost and power. Furthermore, there is a need for a receiver including an analog demodulator and a differentiator that operate at an IF frequency rather than a baseband frequency.